A wingsail is an assembly including one or more aerofoil sections, usually rigid, that is mounted span upright to propel a vessel. European patent specifications 61291, 96554 and 328254 corresponding to U.S. Pat. Nos. 4,467,741, 4,563,970 and 4,856,449 describe various aspects of wingsails, including self-trimming wingsails, of a general type to which the present invention has particular relevance. In such a self-trimming wingsail the angle of attack of a main thrust wing or wings about an upright axis is controlled by an auxiliary control aerofoil or vane, called a tail vane, mounted on a boom extending downwind from the thrust wing. Success with designs incorporating upwind auxiliary control aerofoils has not so far been achieved.
With the prior art self-trimming arrangements incorporating a tail vane, the wingsail assembly has a substantial overall trimming circle which is acceptable on craft with a wide beam, such as multi-hull sailing vessels or on ships where wingsails are used as auxiliary power, when the trimming circle will remain within the plan area of the craft, but on narrower single hulled craft the trimming circle might overhang the gunwales, which is not very practical. One aspect of the present invention is therefore directed towards providing a self-trimming rig with a more compact trimming circle.
Another problem that can arise with self-trimming wingsails is that the movement of the centre of pressure of the main thrust wing as the angle of attack changes, or as the relative deflection of the wing elements are changed, changes the effective turning moment about the main axis of the thrust wing. If the centre of pressure of the main thrust wing is too far from the main axis the compensatory trimming moment required from the auxiliary vane is greater, which in turn leads to a requirement for a larger auxiliary vane and more powerful associated deflection control mechanisms, or to a longer boom and therefore a larger trimming circle. Other aspects of the invention are directed towards controlling the position of the centre of pressure with respect to the main thrust wing axis, and to techniques for minimising the power requirements for auxiliary vane movement.